Cat eye (sclera) a bit red / bloodshot

My cat has a bit of a bloodshot eye (I noticed because he has big eyes and I often see the whites of his eyes when he turns and looks at something).

It's only the left eye, the right one looks pretty normal. Other than that there doesn't seem to be anything unusual around/in the eye area.

Does anyone have any idea what's caused this, or if its something I should be concerned about?

Absolutely. I'll take a picture tonight and post it! Thanks so much!